As a result of their increased performance in terms of output, specific power and power density, nowadays synchronous machines with permanent magnets have extensive application in the field of motor vehicles.
These electrical machines can be produced with a wide range of powers and speeds, and have applications both in vehicles of the “all-electric” type, and in vehicles based on CO2 of the so-called “mild-hybrid” and full-hybrid” types.
The “mild-hybrid” applications generally concern electrical machines of approximately 8 to 10 kW, for example an electric motor which is fitted to the front of the thermal engine, and is coupled to the latter by a drive belt. By means of an electric motor of this type, it is possible to reduce the capacity of the thermal engine (“engine downsizing”) by providing electric torque assistance which supplies additional power, in particular during revving up. In addition, traction at low speed, for example in an urban environment, can also be ensured by this same electric motor.
Another example of an application of electrical machines in this power range consists of driving a centrifugal compressor of a double boosting system of a thermal engine. At low speed the electric compressor assists the turbo compressor which is driven by the exhaust gases, and makes it possible to dispense with an additional step in reduction of the capacities.
Applications of the “full-hybrid” type generally concern 30 to 50 kW motors for architectures of the series and/or parallel type, with a better level of integration of the electric motor(s) in the traction chain of the vehicle.
The remarkable performance levels of the present machines with permanent magnets are largely due to the development of rare earth magnets such as magnets of the neodymium-iron-boron (NeFeB), samarium-iron (SmFe) or samarium-cobalt (SmCo) type, which can have remanence in excess of a tesla.
However, machines with permanent magnets comprising a rotor with a so-called “flux concentration” structure have long since made it possible to obtain substantial magnetic fluxes using magnets with lower remanence, for example magnets obtained from sintered or bonded ferrites.
Also dating back a long time, the dimensional and magnetic characteristics of this type of structure have been optimised, either by undertaking many tests, or more recently by carrying out computer simulations, such as to improve the electric output of the machines.
An example of dimensional optimisation of the magnets and magnetic poles of a rotor with permanent magnets was disclosed in 1971 in the patent of invention FR 2.084.279.
The dimensional optimisation of the magnetic poles has recently come to the forefront of attention once more as a result of rare earth magnets becoming more expensive, because of an unfavourable geopolitical situation.
Since the use of rare earth magnets in a rotor of an electrical machine designed for motor vehicle applications is no longer economically viable, and probably not long-lasting, the other alternative consists of magnets based on ferrites.
However, since the remanence of a ferrite is lower than that of a rare earth magnet, the replacement of rare earth magnets by ferrites leads to a machine with lower performance levels.